A study of the potential inhibition of rat bladder carcinogenesis by 13-cis-retinoic acid is proposed. The carcinogen will be N-Butyl-(4-Hydroxybutyl) nitrosamine administered by gastric intubation twice per week for 6 weeks for a toal dose of 1200 mg. The retinoid will be given continuously in the feed at 300 mg/kg of diet, and beginning either prior, simultaneously and following carcinogen exposure. Possible inhibitory effects will be evaluated by sequential pathologic observation throughout the animal's lifespan and by evaluation of the levels of DNA, RNA and protein synthesis in the urothelium. Specific attention will be given to cell proliferation, tumor incidence, progression or reversibility, histologic grading, invasion and metastases. In order to assess the feasibility and safety of 13-cis-retinoic acid as an anti-neoplastic agent, subchronic toxicity will also be assessed in a 180 day feeding study at doses of 300, 600, and 900 mg/kg of diet. Complete clinical, pathological and biochemical values will be assessed as well as tissue bioaccumulation.